The need to remove sulfur dioxide from gaseous streams, such as flue gases from power plants using fossil fuels, in order to protect the environment has become increasingly important with the enactment of legislation limiting the amount of sulfur dioxide that can be discharged from facilities. In order to meet this need, various sulfur dioxide removal processes have been developed, including dry removal systems and wet scrubbing systems. In wet scrubbing systems, aqueous slurries of reactant products result which are normally dewatered and the dewatered slurry disposed of as a landfill material.
In order to recover various components of an aqueous sludge from a sulfur dioxide wet scrubbing system, attempts have been made to convert the sludge components to saleable byproducts. In systems where lime or limestone are used and calcium sulfites and/or sulfates are present in the sludge, for example, processes have been developed to produce gypsum in a form that would be saleable for use in wallboard or other products using gypsum.
Certain improved sulfur dioxide wet scrubbing systems are based on the use of a lime-magnesium content scrubbing liquor or other magnesium-containing scrubbing liquor which is regenerated through the use of lime. U.S. Pat. No. 3,919,393 and 3,919,394 to Joseph Selmeczi, both of which are incorporated by reference herein, and both of which are assigned to the assignee of the present invention, for example, use a lime slurry containing a specified amount of magnesium components which results in an increased removal of sulfur dioxide from a gaseous stream. In addition, U.S. Pat. No. 5,039,499, issued to Donald Stowe, Jr., which is incorporated by reference herein and which is assigned to the assignee of the present invention, describes a process where magnesium hydroxide is added to an aqueous scrubbing liquor to remove sulfur dioxide from a gaseous stream, with effluent from the scrubber oxidized and then treated with a magnesium-containing lime slurry to obtain regenerated magnesium hydroxide for recycle to the scrubbing unit.
In such processes where a magnesium component is used to remove sulfur dioxide from flue gases, through formation of magnesium sulfite and oxidation of the magnesium sulfite to magnesium sulfate, and lime used to regenerate magnesium hydroxide, calcium sulfate is present which is difficult to separate from the magnesium hydroxide so produced. U.S. Pat. No. 4,996,032 issued to Donald Stowe, Jr. and Lewis Benson, which is incorporated by reference herein and is assigned to the assignee of the present invention, discusses the problems of separating magnesium hydroxide from gypsum and teaches a process for forming a magnesium hydroxide product from an aqueous slurry resulting from a wet scrubbing process.
In systems where magnesium components are present in a wet scrubbing slurry for sulfur dioxide removal, with magnesium sulfite produced that is then oxidized to magnesium sulfate, and the sulfate contacted with lime to produce calcium sulfate as gypsum and magnesium hydroxide, the resultant magnesium hydroxide produced is contaminated with gypsum. The resultant magnesium hydroxide can be partially purified by physical separation, such as froth floatation, elutriation or hydrocloning but, in such cases, the magnesium hydroxide will still retain an amount of gypsum as a contaminant that limits the uses to which such magnesium hydroxide may be put. Generally, we have found that following such physical separation, the magnesium hydroxide separated portion can contain between about 20-45 percent contaminants, a major portion of which is calcium sulfate, or gypsum. In order to purify the magnesium hydroxide so separated, various purification processes may be used but such processes contain use of additional chemicals or expensive equipment and the production of various by-products or the like which must be discarded.
It is an object of the present invention to provide a process for removing sulfur dioxide from flue gases using an aqueous scrubbing medium containing magnesium components and collecting purified magnesium hydroxide and gypsum from the effluent discharged from a wet scrubbing unit used in the process.